1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to an in-plane switching liquid crystal display (IPS-LCD) device and method.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite alignment as a result of their long, thin shapes and are arranged to have initial pretilt angles. The alignment direction can be controlled by applying an electric field. Specifically, variations in an applied electric field influence the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling the applied electric field, an image that has a desired brightness can be produced.
Of the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superior ability in displaying moving images.
In general, a liquid crystal display (LCD) device includes two substrates, which are spaced apart and face each other, and a liquid crystal layer interposed between the two substrates. Each of the substrates includes an electrode, and the electrodes of each substrate also face each other. A voltage is applied to each electrode, and an electric field is induced between the electrodes. An arrangement of the liquid crystal molecules is changed by varying the intensity of the electric field.
However, since the electrodes are positioned on the two substrates, respectively, the electric field induced between the electrodes is perpendicular to the lower and upper substrates. Accordingly, the related art LCD devices have a narrow viewing angle because of the longitudinal electric field.
In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. An IPS-LCD device includes a pixel electrode and a common electrode on the same substrate.
FIG. 1 is a plan view illustrating one sub-pixel of an array substrate for an in-plane switching liquid crystal display (IPS-LCD) device according to the related art.
In FIG. 1, a gate line 14 is formed on a substrate 10 along a horizontal direction in the context of the figure. A data line 30 is formed on the substrate 10 along a vertical direction in the context of the figure. The gate line 14 and the data line 30 cross each other to define a pixel region P. A common line 16 is spaced apart from and parallel to the gate line 14. A thin film transistor T is formed at a crossing portion of the gate line 14 and the data line 30. The thin film transistor T includes a gate electrode 12, a semiconductor layer 22, a source electrode 26, and a drain electrode 28.
In the pixel region P, a pixel electrode 32 is formed. The pixel electrode 32 is connected to the drain electrode 28 and includes portions parallel to the data line 30. In the pixel region P, a common electrode 18 is also formed. The common electrode 18 includes parts extending from the common line 16 along the vertical direction in the context of the figure. The parts of the common electrode 18 alternate with the portions of the pixel electrode 32.
Accordingly, in the IPS-LCD device, liquid crystal molecules are driven by a horizontal electric field induced between the pixel electrode 32 and the common electrode 18 to thereby produce an image.
FIG. 2 is a cross-sectional view illustrating an IPS-LCD device according to the related art. The IPS-LCD device includes an array substrate and a color filter substrate with a liquid crystal layer interposed therebetween.
More particularly, as shown in FIG. 2, a pixel region P is defined on a first substrate 10. A thin film transistor T, as a switching element, is formed in the pixel region P on the first substrate 10, and a common electrode 18 and a pixel electrode 32 are also formed in the pixel region P. The thin film transistor T includes a gate electrode 12, a semiconductor layer 22, a source electrode 26, and a drain electrode 28. The common electrode 18 includes a plurality of parts, and the pixel electrode 32 includes a plurality of portions, which alternate with the plurality of parts of the common electrode 18.
A second substrate 40 is spaced apart from the first substrate 10. A black matrix 42 is formed on an inner surface of the second substrate 40, i.e., a surface facing the first substrate 10. The black matrix 42 corresponds to the thin film transistor T. A color filter layer is also formed on the inner surface of the second substrate 40, and the color filter layer includes three color filters of red 44a, green 44b, and blue (not shown). The color filter layer corresponds to the pixel region P.
A liquid crystal layer 50 is interposed between the first substrate 10 and the second substrate 40. Liquid crystal molecules of the liquid crystal layer 50 are arranged parallel to the first and second substrates 10 and 40.
A lower polarizer 62 is disposed at an outer surface of the first substrate 10 opposite to the thin film transistor T, the pixel electrode 32 and the common electrode 18. An upper polarizer 64 is disposed at an outer surface of the second substrate 40. A light transmissive axis of the lower polarizer 62 is perpendicular to a light transmissive axis of the upper polarizer 64.
FIG. 3 is a schematic view illustrating optical elements changing polarization of light in FIG. 2.
As shown in FIG. 3, the lower polarizer 62, the liquid crystal layer 50, and the upper polarizer 64 change polarization of light.
Each of the lower polarizer 62 and the upper polarizer 64 includes a poly-vinyl alcohol (PVA) film 62a or 64a as a linear polarization element. Each of the lower polarizer 62 and the upper polarizer 64 further includes inner and outer tri-acetyl cellulose (TAC) films 62b and 62c or 64b and 64c at both sides of the PVA film 62a or 64a. The inner TAC film 62b or 64b is adjacent to the liquid crystal layer 50.
In the above IPS-LCD device, a rubbing direction of the liquid crystal layer 50 is parallel to the light transmissive axis of the lower polarizer 62 and is perpendicular to the light transmissive axis of the upper polarizer 64 to thereby provide normally black mode. That is, when a voltage is not applied, a light passing through the lower polarizer 62 is not changed in its polarization while transmitting the liquid crystal layer 50 and reaches the upper polarizer 64. Since the light transmissive axis of the upper polarizer 64 is perpendicular to the light transmissive axis of the lower polarizer 62, the light does not pass through the upper polarizer 64. Therefore, a black image is produced.
In the normally black mode, while a full black image is observed when a liquid crystal panel is seen from the front, a color shift occurs when the liquid crystal panel is seen from the side. This is because the inner TAC film 62b or 64b has a Z-direction retardation, that is, Rth={(nx+ny)/2−nz}×d, wherein nx, ny and nz are X, Y, and Z-directional refractive indexes of the inner TAC film 62b or 64b, respectively, and d is a thickness of the inner TAC film 62b or 64b, wherein the X-direction and the Y-direction are perpendicular to each other and are parallel to the liquid crystal panel, and the Z-direction is perpendicular to the liquid crystal panel. For example, the inner TAC film 62b or 64b may have retardation of about −40 nm. Accordingly, the color shift occurs much stronger as the viewing angle is far from the front.
FIG. 4 is a view illustrating a Poincare sphere showing polarization states of light passing through the optical elements of FIG. 3. The Poincare sphere of FIG. 4 represents polarization states of light when a liquid crystal panel including the optical elements of FIG. 3 is seen from the side.
Generally, in the Poincare sphere, points on the equator A1 indicate linearly polarized light, and the upper and lower poles A2 and A3 represent left and right-handed circularly polarized light, respectively. Points on the upper hemisphere B1 represent left-handed elliptically polarized light, and points on the lower hemisphere B2 represent right-handed elliptically polarized light.
When two points on the equator A1 are symmetrical with respect to the center O of the Poincare sphere, the two points have opposite polarization properties. That is, if an arbitrarily chosen point on the equator A1 designates horizontal polarization, the diametrically opposite point designates vertical polarization.
In FIG. 4, the point S1 on the equator A1 designates a polarization state of a linearly polarized light passing through the PVA film 62a of the lower polarizer 62 when the liquid crystal panel is seen from the front. However, when the liquid crystal panel is seen from the side, the polarization state of the linearly polarized light passing through the PVA film 62a of the lower polarizer 62 is observed on the point S2 on the equator A1. Then, the linearly polarized light passes through the inner TAC film 62b of the lower polarizer 62. If the inner TAC film 62b of the lower polarizer 62 does not have retardation, the polarization state of the linearly polarized light does not change on the point S2 after passing through the inner TAC film 62b of the lower polarizer 62. However, since the inner TAC film 62b of the lower polarizer 62 has retardation of about −40 nm, the linearly polarized light passing through the inner TAC film 62b of the lower polarizer 62 is converted into a right-handed elliptically polarized light on the point S3. The right-handed elliptically polarized light is converted into a left-handed elliptically polarized light on the point S4 after passing through the liquid crystal layer 50 because the liquid crystal layer 50 has retardation Δn·d. Next, the left-handed elliptically polarized light passes through the inner TAC film 64b of the upper polarizer 64. The light passing through the inner TAC film 64b of the upper polarizer 64 has the polarization state on the point S5 around the equator A1 and then reaches the PVA film 64a of the upper polarizer 64.
On the points S4 and S5, the polarization states for R, G, and B lights still are not coincident at one point. Accordingly, a color shift occurs. The color shift increases as the viewing angle is far from the front.
FIG. 5 is a chromatic diagram showing color shift properties of an IPS-LCD device according to the related art. In FIG. 5, colors are widely distributed.
This means that the color shift occurs in a wide range of viewing angles. The color shift is caused by the inner TAC films 62b and 64b of the lower and upper polarizers 62 and 64 having the retardation of about −40 nm. The color shift has bad effect on a wide viewing angle of the IPS-LCD device.